HashMap：线程不安全效率高，允许null值null键 Hashtable 线程安全效率低，不允许存null值null键
HashMap<String, String> map = new HashMap<>(); map.put(null,null);//允许null值null键 Hashtable<String, String> table = new Hashtable<>(); //table.put(null,"abc");//不允许存null值null键 //table.put("aaa",null);不允许存null值null键 //table.put(null,null);不允许存null值null键 //HashSet:底层用HashMap 存储 HashSet<Integer> integers = new HashSet<>(); integers.add(100);